Method for making thin films in metal/ceramic composite

ABSTRACT

The invention concerns a method for making thin films in metal/ceramic composite, characterised in that it consists in a) preparing a suspension (S) in an organic solvent from a substantially homogeneous mixture of ceramic reinforcements of metal particles, a binder, a plasticizing agent and an organic dispersant, the metal particles constituting at least  5  wt. % of the suspension; b) tape casting the suspension (S) to form a thing film (B), then climinating organic compounds contained in the binder and the plasticizing agent from said thin film; c) densifying the thin film from which said organic compounds have been removed in an oven.

FIELD OF THE INVENTION

[0001] The invention relates to a process for manufacturing thin filmsmade of metal/ceramic composite in which the ceramic reinforcements arehomogeneously distributed in the metallic matrix.

[0002] The invention is used for applications in all fields usingsubstrates or films made of a metal/ceramic composite and particularlyfor manufacturing electronic components, for example designed for theautomobile or aeronautics field.

BACKGROUND OF THE INVENTION

[0003] Conventionally, metallic films are made by a rolling process.However, this rolling process cannot provide a uniform distribution ofceramic reinforcements within the metallic matrix; therefore, it is notsuitable for manufacturing metal/ceramic composite films. Furthermore,residual stresses caused by rolling can cause cracking of the film ifthe concentration of ceramic powder is above a given value.

[0004] Other processes used for making metal/ceramic composite filmsinclude pressing, injection and extrusion processes. None of theseprocesses is capable of obtaining film with an excellent surfacecondition. Furthermore, these processes become expensive if the film hasto be less than one millimetre thick.

[0005] There are also tapes casting processes for making thin films.This type of process is described in articles by ALCOCK J., DESCRIBE S.,Tape casting, a flexible approach to surface engineering, MaterialsWorld, 13-14, February (2000), BÖHNLEIN-MAUβ J., SIGMUND W., WEGNER G.,MEYER W. H., HEβEL F., SEITZ K, ROOSEN A., The function in the tapecasting of alumina, Advanced Materials, vol. 4, No. 2, 73-81 (1992);MORENO R., The role of slip additives in tape casting technology: partI—Solvents and dispersants, American Ceramic Society Bulletin, vol. 71,No. 10, 1 521-1 531 (1992); MORENO R., The role of slip additives intape casting technology: part II—Binders and Plasticizers, AmericanCeramic Society Bulletin, vol. 71, No. 11, 1 647-1 657 (1992) andpatents U.S. Pat. No. 5,002,710 and U.S. Pat. No. 5,473,008.

[0006] However, despite all the work done on tape casting, there is nodevice (bench+suspension) capable of casting tapes made of metal/ceramiccomposite material.

SUMMARY OF THE INVENTION

[0007] The purpose of the invention is precisely to overcome problemswith processes for manufacturing thin films described previously. Toachieve this, it proposes a process for making thin films made of ametal/ceramic composite using a tape casting method.

[0008] More precisely, the invention relates to a process formanufacturing composite metal/ceramic thin films, consisting of:

[0009] a) preparing a suspension (S) in an organic solvent starting froma substantially homogenous mixture of ceramic reinforcements, metallicparticles, a binder, a plasticizer and a dispersant, the metallicparticles constituting at least 5% by weight of the suspension;

[0010] b) tape casting the suspension to form a thin film, and thende-binding said thin film;

[0011] c) densifying the de-binded thin film in a furnace.

[0012] The tape casting technique allows to orient and control thedistribution of ceramic reinforcements.

[0013] In other words, the process described according to the inventionis a means of making composite metal/ceramic films with an orientationof ceramic particles in the plane of the film, particularly for highlyanisotropic particles, like fibres and platelets. This allows to improvesome properties of the composite in the plane of the film, such asreducing the coefficient of thermal expansion and increasing the thermalconductivity.

[0014] Advantageously, the viscosity of the suspension is between 0.5and 3 Pa.s.

[0015] Preferably, the suspension is made by mixing:

[0016] at least one metallic powder and at least one ceramicreinforcement constituting about 30 to 60% of the total volume of thesuspension;

[0017] an organic solvent constituting about 15 to 45% of the volume ofthe suspension;

[0018] a binder and a plasticizer constituting about 30 to 70% of thevolume of the suspension;

[0019] a dispersant representing about 0.1 to 2% by weight of theceramic and metallic powders; and

[0020] additives representing about 0.01 to 2% of the mass of metallicand ceramic powders.

[0021] According to one embodiment of the invention, the dispersant is aphosphoric ester, a polyacrylate, a sulfonate, a perfluorate or an acidwith a carbon chain having 2 to 30 atoms of carbon.

[0022] According to the invention, the metallic powder may be a copper,aluminium, silver, gold, nickel, titanium, chromium or zinc powder, oran alloy of two or more of these materials. The ceramic reinforcementmay be a powder and/or a short fibre (i. e. a fibre which length rangesfrom 1 and 500 μm) of graphite, carbides, nitrides or oxides.

[0023] According to one variant of the invention, densification of thefilm consists in sintering the film in a furnace.

[0024] According to another variant of the invention, densification ofthe film consists in hot rolling and annealing the film.

[0025] Preferably, the preparation of the suspension consists in:

[0026] grinding the metallic powders and ceramic reinforcements in a jarmill or by attrition with the solvent and dispersant; then

[0027] adding and mixing a binder and a plasticizer to this substance.

[0028] The invention also relates to a process for preparing compositeparts with a laminated structure in which several thin films (“green”films) are formed by steps a) and b) described above, and said thinfilms are then stacked and the stack is subjected to thermocompression.

[0029] Preferably, the compositions of the stacked thin films aredifferent.

BRIEF DESCRIPTION OF THE FIGURES

[0030]FIG. 1 diagrammatically shows the suspension preparation stepstarting from metallic particles and ceramic reinforcements;

[0031]FIG. 2 diagrammatically shows the tape casting step of thesuspension to form a thin film; and

[0032]FIGS. 3A and 3B show two embodiments of the film densificationstep in a furnace.

DETAILED DESCRIPTION OF EMBODIMENTS

[0033] The invention relates to a process for making thin films made ofa metal/ceramic composite.

[0034] This process consists of preparing a suspension, also called a“slurry” (“barbotine” in French), comprising a substantially homogenousmixture of ceramic reinforcements and metallic particles.

[0035] These metallic particles and ceramic reinforcements are chosen inthe form of one or several metallic powder(s) and one or several ceramicreinforcement(s), respectively. These powders and short fibres are mixedwith an organic solvent, a dispersant, a binder and a plasticizer.

[0036] The amount of these various elements is as follows:

[0037] metallic powders and ceramic reinforcements represent 30 to 60%of the total volume of dry matter in the suspension (in other words ofthe entire volume occupied by the binder, the plasticizer, thedispersant and metallic and ceramic powders);

[0038] the solvent represents 15 to 45% of the total volume of drymatter;

[0039] the binder and the plasticizer represent 30 to 70% of the volumeof dry matter;

[0040] the dispersant represents between 0.01 and 2% of the mass ofmetallic powders and ceramic reinforcements;

[0041] other additives are added such as release agents and/or wettingagents, that represent between 0.01 and 2% of the mass of metallic andceramic powders.

[0042]FIG. 1 shows this first step in the process according to theinvention, namely the suspension preparation step.

[0043] This step for preparation of the suspension S consists firstly ingrinding the metallic and ceramic powders with the solvent and thedispersant, in a jar or by attrition. This grinding step is done usingan attrition grinder (represented by reference 1 in FIG. 1) or by a jarmill.

[0044] The assembly thus obtained is then mixed with binders andplasticizers using a mixer, reference 2.

[0045] For example, the metallic powder(s) that will form the metallicmatrix of the suspension may be a copper, aluminium, silver, gold,nickel, titanium, chromium or zinc powder, or a powder of an alloy oftwo or more of these metals.

[0046] The ceramic reinforcement(s) that will form the ceramicreinforcements of this suspension may for example be a graphite powderor a short graphite fibre, or a powder or a short fibre based oncarbides such as silicon carbide, or nitrides such as aluminium nitride,or oxides such as silica or zirconium tungstate.

[0047] Ceramic reinforcements may be in the form of fibres or plateletsor substantially spherical grains with a diameter of between about 0.1μm and 100 μm.

[0048] Fibres are usually short fibres with a diameter of 10 nm to 10μm, and with a length of 100 nm to 10 nm.

[0049] These ceramic reinforcements may be coated with a layer ofmetallic material such as cobalt, nickel, silver or gold. In this case,the thickness of the metallic coating is at least 0.01 μm. This coatingmay be achieved by immersion of the ceramic reinforcements in anelectrolytic bath. The advantage of this coating is that it improvesmaterial densification during the film densification step, and inparticular when this densification consists in sintering, since itincreases the metal/ceramic interface.

[0050] The suspension used according to the invention is an organicsuspension or system.

[0051] Thus, the solvent used to make this suspension S is an organicsolvent, usually chosen from among cetones, alcohols and mixturesthereof.

[0052] The function of the dispersant used to make this suspension is tomake the suspension homogenous and stable by creating repulsion forcesbetween the ceramic reinforcements and the metallic particles.

[0053] In other words, the dispersant enables good stability and gooddispersion of the particles among each other. The dispersant enablesproduction of a homogenous and compact tape after drying.

[0054] This dispersant is chosen from among surfactants, macro-moleculeslike fish oil, phosphoric esters, polyacrylates, sulfonates,perfluorates and acids with a carbon chain having 2 to 30 atoms ofcarbon, such as for example oxalic acid and stearic acid.

[0055] The binder used to make this suspension plays the role ofunparting cohesion to the tape (or film) after the solvent hasevaporated. This binder is usually a compound that is non-soluble inwater and is chosen from among polyalcohols, vinyl compounds, such aspolyvinyl-butyral, and acrylic compounds and mixtures thereof.

[0056] The plasticizer used in this suspension plays the role ofunparting good flexibility and good fluidity to the tape; thisflexibility is necessary when the suspension is being tape cast andlater when handling the tape. For example, this plasticizer may be apolyethylene glycol or dibutylphthalate.

[0057] In other words, the suspension also contains a plasticizer toobtain a flexible, sufficiently strong, green tape, or thin film, sothat it can be handled. The binder/plasticizer ratio is a means ofadjusting the mechanical cohesion and flexibility of the tape.Therefore, these tapes can be stacked and thermocompressed so as to makestacks of tapes with different compositions. This solution cannot beachieved with processes according to prior art.

[0058] Note also that the system and the suspension used according tothe invention do not require any lubricant.

[0059]FIG. 2 diagrammatically shows the second step of the processaccording to the invention, in other words the tape casting step of thesuspension. The suspension S made during the first step is cast onto acasting bench 3 so as to form a tape B, also called a thin film. Tapecasting consists of casting the suspension S on a support that may forexample be a steel tape 8 or a polymer wire, reference 5 in FIG. 2. Theviscosity of the suspension must be of about 0.5 to 3 Pa.s to facilitatecasting of the suspension.

[0060] The suspension is cast by creating a relative movement between ashoe 6 on the casting bench and the support 5. The shoe 6 is providedwith knives 7 with an adjustable height. Thus, the film thickness can bemodified by changing the height between these knives 7 and the support5. Thus, a very uniform film thickness can be obtained using this tapecasting method.

[0061] When the suspension S has been cast in the form of a tape B, thetape B is passed through a drier 4 under a controlled atmosphere, toeliminate organic compounds. This step is called “de-binding”. Moreprecisely, thermal de-binding consists of gently heating the tape ofmaterial under a controlled atmosphere in a furnace or dryer 4 in orderto eliminate the contained organic compounds, mainly the binder and theplasticizer. For example, the heating rate in the drier is about 0.2 to2° C./minute between 100° C. and 500° C.

[0062]FIGS. 3A and 3B show two different embodiments of the third stepof the process according to the invention, in other words the filmdensification step.

[0063] This densification step consists in evaporating the solvent anddrying the thin film obtained after de-binding.

[0064] The purpose of this film densification step is to evaporate thesolvent. For example, this may be done in two different ways: the filmmay be densified by sintering in a passage furnace or in a discontinuousfurnace, or by hot rolling using a roll and an annealing furnace.

[0065] The first variant shown in FIG. 3A shows that the film B obtainedafter de-binding is cut into plates P1 to Pn. These plates are insertedin a furnace reference 9 under a controlled atmosphere. This furnace maybe a passage furnace or a discontinuous furnace. Densification bysintering is done under a controlled atmosphere, or under a reducingatmosphere, for example such as hydrogen, hydrogenated nitrogen, argonor hydrogenated argon, in order to prevent oxidation of the material.

[0066] The sintering temperature depends on the particle size and natureof the metallic powders and ceramic reinforcements. For example, for ametallic copper powder, the temperature is between 700° C. and 1080° C.;for aluminium, the temperature is between 450° C. and 650° C.

[0067] The second variant of the densification step is shown in FIG. 3B.In this variant, the film B is inserted in a roll 10 inside an annealingfurnace 11. The film B is then hot rolled in the furnace 11 under acontrolled atmosphere. The film B is cut into plates P1, P2, . . . atthe exit from the annealing furnace 11.

[0068] This film hot rolling and annealing densification method allowsto improve the densification of the material under the action ofpressure and temperature. Therefore this variant is particularlysuitable for metal/ceramic composites that are not well densified bynatural sintering and for composites constituted by ductile metals likecopper, aluminium or gold.

1. Process for manufacturing composite metal/ceramic thin films,consisting of: a) preparing a suspension (S) in an organic solventstarting from a substantially homogenous mixture of ceramicreinforcements, metallic particles, a binder, a plasticizer and adispersant, the metallic particles constituting at least 5% by weight ofthe suspension; b) tape casting the suspension to form a thin film, andthen de-binding said thin film; c) densifying the de-binded thin film ina furnace.
 2. Process according to claim 1, characterised in that theviscosity of the suspension (S) is between 0.5 and 3 Pa.s.
 3. Processaccording to claim 1 or 2, characterised in that the suspension (S) ismade by mixing: at least one metallic powder and at least one ceramicreinforcement constituting together about 30 to 60% of the total volumeof the suspension; an organic solvent constituting about 15 to 45% ofthe volume of the suspension; a binder and a plasticizer constitutingabout 30 to 70% of the volume of the suspension; a dispersantrepresenting about 0.1 to 2% by of the weight of the ceramic andmetallic powders; and additives representing about 0.01 to 2% of themass of metallic and ceramic powders.
 4. Process according to any one ofclaims 1 to 3, wherein the binder is a compound that is non-soluble inwater chosen from among polyalcohols, vinyl compounds, acrylic compoundsand mixtures thereof.
 5. Process according to any one of claims 1 to 4,in which the organic solvent is chosen from among cetones, alcohols andmixtures thereof.
 6. Process according to any one of claims 1 to 5, inwhich the dispersant is chosen from among surfactants, macro-moleculeslike fish oil, phosphoric esters, polyacrylates, sulfonates,perfluorates and acids with a carbon chain of 2 to 30 carbon atoms. 7.Process according to any one of claims 3 to 6, characterised in that themetallic powder is a copper, aluminium, silver, gold, nickel, titanium,chromium or zinc powder, or an alloy of two or more of these materials.8. Process according to any one of claims 3 to 7, characterised in thatthe ceramic reinforcement is a powder and/or a short fibre of graphite,carbides, nitrides or oxides.
 9. Process according to any one of claims1 to 8, characterised in that densification of the film consists insintering the film in a furnace.
 10. Process according to any one ofclaims 1 to 8, characterised in that densification of the film consistsin hot rolling and annealing the film.
 11. Process according to any oneof claims 3 to 10, characterised in that preparation of the suspensionconsists of: grinding the metallic and ceramic powders with the solventand dispersant in a jar mill or by attrition; then adding and mixing abinder and a plasticizer to this substance.
 12. Process for preparing acomposite part having a laminated structure in which several thin filmsare formed according to steps a) and b) described in claim 1, and saidthin films are stacked and said stack is subjected to thermocompression.13. Process according to claim 12, wherein the stacked thin films havedifferent compositions.